1. Field of the Invention
The invention generally relates to the synthesis of metal organic framework (MOF) molecules. In particular, the invention concerns rapid, simple, and versatile methods of producing MOF molecules using microwave assisted synthesis.
2. Related Art
MOFs are organometallic nanoporous structures with high surface area and tailorable selectivity, which makes them suitable for a wide range of applications. They may have a cubic crystalline structure that is formed by copolymerization of metals or metal oxides with organic ligands, resulting in metal-oxide clusters connected by organic linkers. FIG. 1 is a diagram of a typical MOF's 10 crystalline structure including metal or metal oxides, here shown as polyhedrons 12, having polymer ligands 14 extending between them. This highly ordered structure facilitates the creation of interior pores and channels. MOFs are known to have 1-3 nanometer (nm) pores. FIG. 2 is a view of the structure and channels 16 of one of the new MOFs discovered by Applicants and disclosed herein, Zn-MOF-1, and in pending U.S. patent application Ser. No. 11/539,905, filed Oct. 6, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety. As shown in FIG. 2, Zn-MOF-1 has channels 16 of about 10 Å×10 Å. MOF crystals are known to have an average diameter of about 200 microns.
MOFs have been the focus of intense activity in recent years because of their extremely high porosity and tailorable molecular cavities. The surfaces of MOFs' pores and channels increase the overall surface areas of MOFs allowing them to have high porosity that is comparable to or larger than that of zeolites. For example, MOF-5, a known MOF, may have a surface area of about 2900 m2/g and IRMOF-177 may have a surface area of about 4500 m2/g. The high surface area of MOFs allows them to be used in a wide range of applications. For example, MOFs have been studied for a variety of applications including hydrogen storage, selective sorption, non-linear optical materials, templates for the creation of molecular species architectures, and as catalysts e.g., catalysts to store H2 (in fuel cells) or CO2.
Applicants have discovered that MOFs have properties that make them highly advantageous as preconcentrators including, for example, high sorption capacity due to their high surface area, high selectivity to specific analytes, inert nature which does not decompose the analyte, thermal stability, and result in unexpectedly high gains in detection. Accordingly, MOFs are used to selectively sorb specific analytes in a preconcentrator. For example, MOFs may be used in particle form, or they may be incorporated into a film inside the preconcentrator. Once the analytes are fully sorbed by the MOFs, the analytes are released by thermal desorption, for example. Then, they can be purged and transferred from the preconcentrator to a detector, for example. The structure and properties of MOFs that make them highly suitable for use as selective sorbents in preconcentrators are discussed below. Details about use of MOFs in a variety of preconcentrators may be found in the above-noted U.S. patent application Ser. No. 11/539,405 filed Oct. 6, 2006.
Numerous methods have been developed for synthesizing MOFs using precursors including a metal precursor and corresponding organic spacing ligand. Solvothermal and hydrothermal synthesis methods using these precursors have conventionally been employed to form MOF crystals. Solvothermal synthesis is a method where precursors for MOF crystal formation are heated in a solvent other than water. In hydrothermal synthesis, precursors for MOF crystals are heated in water. Hydrothermal synthesis is suitable when the ligand precursor is soluble in water. In both conventional solvothermal and hydrothermal synthesis, a solution with MOF precursors is typically maintained at a predetermined equilibrium temperature for an extended period to induce crystallization. Solvothermal and hydrothermal synthesis methods are typically slow, often taking hours and even days.
General information on different known MOFs and conventional synthesis methods are reported in a number of publications, including, “Design and Synthesis of an Exceptionally Stable and Highly Porous Metal-Organic Framework,” Yaghi et al., Nature 402 (1999) 276-279; “Interwoven Metal-Organic Framework on a Periodic Minimal Surface with Extra-Large Pores,” B. Chen, M. Eddaoudi, Yaghi et al. Science 291 (2001) 1021-1023; “Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application and Methane Storage,” Yaghi et al, Science 295 (2002) 469-472; “Reticular Synthesis and Design of New Materials,” Yaghi et al., Nature 423 (2003) 705-714; “Hydrogen Storage in Microporous Metal-Organic Frameworks,” Yaghi et al., Science 300 (2003) 1127-1129.
As mentioned above, reported solvothermal synthesis methods are slow, typically taking a day or more. U.S. Published Application No. 2003/0004364 discloses a solvothermal process to form MOF materials that takes about one day to several days. In the synthesis method of the above-referenced application, a metal salt and a linear ditropic carboxylate are dissolved in a solvent to form a solution. The solution is then crystallized, which involves at least one of leaving the solution at room temperature, adding a diluted base to the solution to initiate the crystallization, diffusing a diluted base into the solution to initiate the crystallization, and transferring the solution to a closed vessel and heating to a predetermined temperature.
A multi-day hydrothermal synthesis process has also been proposed for the production of nonlinear optically active MOF material. (See e.g., “A Novel Optical Metal-Organic NLO Material Based on a Chiral 2D Coordination Network,” Lin, et al., J. Am. Chem. Soc. 1999, 121, 11249-11250.) Others have reported a thermally stable [Cu3(TMA)2(H2O)3]n framework structure produced through a 12 hour solvothermal synthesis. (See e.g., “A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n,” Chui, et al., Science, 1999, 283, 1148.) Catalytic active homochiral metal-organic materials formed by a two-day liquid diffusion method or solvothermal method have been reported. (See e.g., “A Homochiral Meta-organic Porous Material for Enantioselective Separation and Catalysis,” Seo, et al., Nature, 2000, 404, 982-986; “A Homochiral Metal-Organic Material with Permanent Porosity, Enantioselective Sorption Properties, and Catalytic Activity”, Dybtsev, et al., Angew. Chem. Int. Ed., 2006, 45, 916-920.)
Others have reported porphyrin MOF structures formed by a two-day solvothermal synthesis. (See e.g., “A functional zeolite analogue assembled from metalloporphyrins,” Kosal, M. E.; Chou, J.-H., Wilson, S. R.; Suslick, K. S, Nature Materials, 2002, 1, 118-121.) Synthesis by one-week deprotonating vapor diffusion has also been reported. (See e.g., “A Robust Microporous Zinc Porphyrin Framework Solid”, Smithenry, D. W. et al. Inorg. Chem. 2003, 42, 7719.)
Microwave-assisted processes have been used to produce metal particles and oxide particles. Such processes can involve heating a solution with microwaves for a period of an hour or more to produce nano-sized crystals of metal. Typically, metal particle sizes are about 15 nm (Panda A. B.; Glaspell, G. EI-Shall, M. S. J Am. Chem. Soc. 2006, 128 2790; Lu, Q., Gao, F., Li, D., Komarneni, S. J. Mater. 2005 1, 1). Microwave synthesis to provide 5-20 nanometer sized particles of oxides is also known (Tomsett, G. A., Conner, W. C. Yngvesson, K. S. Chem Phys Chem. 2006, 296).
Microwave-assisted methods of synthesizing certain types of framework molecules have been studied. For example, Lin et al. (Zhuojia Lin, David S. Wraggg, and Russell E. Morris, “Microwave-assisted synthesis of anionic metal-organic frameworks under ionothermal conditions,” Chem. Commun., published May 21, 2006, 2021-2023) describes reactions of metal acetate with trimesic acid in 1-ethyl-3-methyl imidazolium bromide (ionic liquid) under microwave or conventional ionothermal conditions resulting in the formation of novel three-dimensional anionic frameworks templated by the ionic liquid solvent. The anionic MOFs described in Lin et al. were synthesized in ionic liquid, which turns to a liquid media (i.e. melts) only when heated over 80° C. In contrast, the MOFs of this invention use common organic solvents such as diethylformamide(DEF), ethanol and water. Further, in Lin et al. the ionic solvent EMIm (1-ethyl-2-methyl imidazolium bromide) becomes building blocks in the framework structure and carries 2+ charge. In contrast, the MOF of this invention is a neutral framework and can be isolated from the solvent. In addition, the microwave processing time for anionic MOF of Lin et al. is 50 mins; the processing time for the MOFs in this invention is about 1 minute.
In addition, Rajic et al. (N. Rajic, D. Stojakovic, N. Zabukovec Logar, and V. Kaucic, “An evidence for a chain to network transformation during the microwave hydrothermal crystallization of an open-framework zinc terephthalate,” J Porous Mater 2006, 13: 153-156) discusses methods for synthesizing a 3-D open framework and a chain structured zinc terephthalate using hydrothermal crystallization under microwave heating at 180° C. The MOFs described in Rajic, however, are linear MOFs that do not have the high surface area, high porosity and tailorable molecular cavities that non-linear MOFs produced by conventional hydrothermal and solvothermal methods have. Moreover, they lack high thermal stability. The MOFs of this invention have a 3-D structure that is more robust than linear MOFs because they have coordination bonds in contrast to linear MOFs which have weak Van der Waal bonding. Therefore, the linear MOFs of Rajic are not suitable for a wide range of applications, as are non-linear MOFs.
There is a need for a method of manufacturing MOFs whereby the MOFs can be easily reproduced. There is also a need for a rapid synthesis method for MOFs because rapidly formed MOFs can be more suitably used as research tools for further evaluation. In addition, there is a need for MOFs having a uniform size and shape as well as MOFs which are smaller in size for certain applications. Furthermore, there is a need for a method to make MOFs that produce uniform seeding conditions that can permit secondary grow processes to form larger crystals for used in certain applications.
Thus, rapid methods for the synthesis of non-linear MOFs that have a wide range of applications have not been developed. Accordingly, there is a need for improved methods to rapidly synthesize non-linear MOFs.